By Sam Kim SEOUL, Dec. 17 (Yonhap) -- North Korea warned Friday that another war with South Korea would involve nuclear arms and spread beyond the peninsula, upping the ante as a prominent U.S. politician and a top U.S. nuclear envoy each visited Pyongyang and Seoul to defuse tension.

Uriminzokkiri, the communist state's official Web site, also said in a commentary that war on the Korean Peninsula is only a matter of time, stoking already high tensions after the North shelled a western South Korean island on Nov. 23 and killed four people.

"If war breaks out, it will lead to nuclear warfare and not be limited to the Korean Peninsula," it said. The Koreas remain technically at war after the 1950-53 Korean War ended in a truce, In a separate editorial carried in the North's ruling party newspaper, Pyongyang reiterated its long-running demand that Washington agree to a peace pact to formally end the war and withdraw its 28,500 troops stationed in South Korea as a deterrent.

The violent rhetoric came as New Mexico Governor Bill Richardson was in North Korea on a private mission aimed at taming the regional tensions heightened by the deadly bombardment and the revelation of a modern uranium enrichment program in the North.

Uranium provides a second track to developing atomic bombs in addition to plutonium. Pyongyang, which has conducted two nuclear tests in 2006 and 2009, claims its uranium enrichment activity is intended for "peaceful" purposes.

U.S. officials have rejected the claim as false. Sung Kim, Washington's envoy for stalled six-party nuclear talks on North Korea, flew to Seoul on Friday from Beijing where he had assisted Deputy Secretary of State James Steinberg in persuading China to use its influence over Pyongyang to rein in the regime.

The trip by Richardson came at the invitation of North Korea's chief nuclear envoy, Kim Kye-gwan. On Thursday, the North's foreign ministry said it supports "all proposals for dialogue, but would never beg for it," including the six-nation talks that also involve the U.S., South Korea, Japan, Russia and China.

The November shelling of the island of Yeonpyeong in the Yellow Sea led to the deaths of two marines and two civilians in the first direct armed attack on South Korean soil since the Korean War ended.

Seoul has vowed to make Pyongyang pay a price, while North Korea claims its side was first provoked by South Korean forces conspiring with the U.S. to invade the communist country.

South Korea and the U.S. dismiss any chances of immediate dialogue with North Korea, while China has proposed emergency six-nation talks on ways to quell the tension. U.S. State Department spokesman Philip Crowley said in Washington that the North should first carry out a series of steps to restore peace in the region and denuclearize itself under a 2005 six-nation agreement.